1. Field of the Invention
The present invention relates to an intake port structure for an internal combustion engine having primary and secondary intake passages for each engine cylinder, and more particularly to such an intake port structure designed to produce swirls of air-fuel mixture in the engine cylinder for improving fuel combustion therein.
2. Prior Art
Internal combustion engines having primary and secondary intake passages for supplying air-fuel mixtures under low and high load conditions have widely been used to improve engine outputs. With such an internal combustion engine having a plurality of cylinders, and when the engine operates under low and medium loads using only the primary intake passage, an air-fuel mixture tends to flow back through secondary intake passages connected to closed intake valves into other secondary intake passages connected to opened intake valves, through which the air-fuel mixture is introduced into combustion chambers. Such an air-fuel mixture leakage through the secondary passages results in a reduced rate at which the air-fuel mixture is drawn from the primary intake passages into the cylinders and hence lessens swirls of air-fuel mixture formed therein.
To prevent the secondary air-fuel mixture leakage, there have heretofore been used check valves or restriction valves for blocking off a backward flow of air-fuel mixture. However, the check valve provides increased resistance to the flow of an air-fuel mixture through the secondary intake passages into the engine cylinders. The restriction valve renders the secondary intake passages quite complex in structure.